Bone conduction is a mechanism for delivering sound to the cochlea by sending vibrations through the skull rather than the eardrum and middle ear as in ordinary air conduction hearing. For patients with conductive hearing loss due to disease or trauma, hearing aids that employ bone conduction offer a promising way of restoring hearing. While hearing aids relying on bone conduction have existed for many years, it was only with the advent of the implantable bone anchored hearing aid (BAHA®) that a reliable, effective and commercially successful option became available. The existence of the BAHA has led to an expansion of the use of bone conduction to treat other hearing disorders. For example, bone conduction has recently been used for patients with single-sided deafness to route acoustic information on the deaf ear side to the hearing ear. For patients with moderate to severe conductive hearing loss, bone conduction technologies offer a promising alternative to traditional air-conduction hearing aids. Bone conduction represents an alternative route for sound to enter the cochlea in a way that completely bypasses the middle ear. As a result, even patients with completely devastated middle ears can benefit from bone conduction technologies.
Sound is transduced into neural impulses at the inner hair cells of the cochlea. Thus in order to achieve hearing, an actuator must have a means for moving these hair cells. In ordinary air-conducted hearing, pressure oscillations in air drive the motion of the tympanic membrane which is connected to the oval window of the cochlea through the middle ear ossicles. The stapes footplate pushes the oval window in and out, driving fluid through the cochlea. The resulting fluid pressure shears the basilar membrane to which the hair cells are attached, and their motion opens ion channels that trigger neural impulses.
When the skull vibrates, a variety of inertial and elastic effects transmit some fraction of those vibrations to the cochlear fluids and thence to the hair cells. While the detailed mechanics of the interaction between vibrations in the skull and the cochlear fluids is an area of active research, it is generally accepted that any motion of the bony cochlear promontory will result in some perception of sound. In designing bone-conduction based hearing aids one typically considers the vibratory level of promontory bone motion as a rough correlate for bone-conducted hearing level. Conversely, any device that can achieve significant motions of the promontory will be a promising candidate for a bone-conducted hearing device.
The BAHA® consists of two parts, a percutaneous titanium abutment that is screwed directly into the patient's mastoid where it osseointegrates in the bone, and an electromagnetic motor that drives a 5.5 g inertial mass, thereby generating a reactive force into the abutment. While popular and effective, the percutaneous nature of the BAHA® often leads to skin infections and patient discomfort, as well as presenting a cosmetic barrier to adoption. The abutment requires constant post-operative care, extensive skin thinning of subcutaneous tissues around it and the removal of hair follicles in its vicinity to function well. For low-frequency vibrations below approximately 1200 Hz, the high stiffness of the skull guarantees that the entire head moves as a rigid body. Consequently the BAHA® must drive the mass of the entire head in order to excite motion of the cochlear fluids in the cochlea. While effective, this whole-head motion requires considerable energy, and a consequent large drain on the battery powering the BAHA®.
A subcutaneous bone conduction implant (BCI) has been reported and validated on embalmed heads. This device relies on an improved version of the BAHA motor called the balanced electromagnetic separation transducer (BEST). The BEST-BCI works on essentially the same principle as the BAHA, relying on an inertial mass reactance to provide the vibratory power. While promising, the device dimensions are large and implantation requires a 15 mm×10 mm×10 mm hole to be made by resectioning of the mastoid. Many mastoids are too sclerotic to accommodate this and many candidate patients who would otherwise conform to indications for bone conduction implants have already undergone extensive mastoid surgery and do not possess intact mastoids suitable for implantation.
Other implantable hearing devices target different parts of the auditory system to treat conductive hearing loss. Middle ear implants such as the Vibrant Sound Bridge are available. While effective, these devices require an intact ossicular chain and the implantation procedure is time-consuming and delicate. More recently, middle-ear implants have been placed in the round window niche of the cochlea where they directly drive the round window membrane causing motion of perilymph. Although this approach is promising where the middle ear is not sufficiently intact for a middle ear implant, the surgery remains quite difficult and results to date have been mixed. Another kind of implantable hearing aid is the cochlear implant, but this is typically indicated only for sensorineural loss, not for conductive loss as its implantation often results in the destruction of residual hearing.
There is, therefore, a need for bone-conduction technologies that can provide vibratory stimulation to the cochlea without percutaneous abutments or invasive and delicate surgical procedures, and that are more efficient than current technologies.